Gas separations, for example oxygen separation from air, may be accomplished by several techniques. Most common among these techniques are cryogenic separation and pressure swing adsorption (PSA). Cryogenic separation depends on differences in boiling temperatures of the gases being separated. Through a series of compression and expansion cycles, air, for example, can be cooled to the point that oxygen (boiling point=-183.degree. C.) condenses. At this temperature, nitrogen (boiling point=-195.8.degree. C.) remains as a gas and thus separation of the liquid nitrogen and the gaseous oxygen can be effected by fractional distillation techniques. Cryogenic separation is, because of the compression and refrigeration requirements, relatively expensive. However, it remains one of the widely used gas-gas separation techniques and is a commercial enterprise.
Pressure swing adsorption is a technique in which a mixture of gases is passed over and/or through a sorbent material such as activated carbon at high pressure. Depending on the gas mixture, selective sorption of one of the gases can occur, thus depleting the stream of that gaseous species. The sorbed gas is then desorbed by lowering the pressure of the sorbent system (thus, the system pressure "swings" from a high to low pressure), enriching the gas stream in the sorbed species. Pressure swing adsorption is a mature technology and is economically favorable in some gas-gas separations, especially in smaller-scale systems. However, significant improvements in the process and process economics are needed.
Other techniques using sorbents may also be used to separate gases. For example, variations in temperature may be used to affect sorption and desorption of gases. Generally, gases may be sorbed at lower temperatures and by increasing temperature, the gases are desorbed. Thus, a temperature swing would be used to operate the system.
Two mechanisms, diffusion and sorption, appear to be most important in gas separations using sorbents. Consider, for example, the nitrogen-oxygen system. In a PSA process, an oxygen molecule having a smaller diameter (.about.3.4 .ANG.) than a nitrogen molecule (.about.3.6 .ANG.) diffuses much more rapidly than nitrogen into the sorbent pores. This difference in diffusion rate provides one of the base for separation using sorbents. The second mechanism relates to the strength of the sorption forces between the sorbent and the gases being sorbed. In the nitrogen-oxygen system, oxygen is more strongly sorbed than is nitrogen. The enhancement of these two processes in gas separation systems using sorbents is the aim of the subject invention.